This invention relates to a multiple beam raster output scanning system and, more particularly, to a multiple beam raster output scanning system with time division multiplexing lasers and dynamic beam separators.
In xerographic printing (also called electrophotographic printing), a latent image is formed on a charged photoreceptor, usually by raster sweeping a modulated laser beam across the photoreceptor. The latent image is then used to create a permanent image by transferring and fusing toner that was electrostatically attracted to the latent image onto a recording medium, usually plain paper.
Typically, a laser beam is deflected from a rotating mirror to sweep the beam across the photoreceptor. The rotating polygon mirror and related optics are generally referred to as Raster Output Scanners (ROSs). Printers that sweep several beams simultaneously are referred to as multiple beam printers.
Problems arise in xerographic printing when attempting to print at high speed, particularly when printing in color.
A color xerographic printer requires a separate image for each color printed, hereinafter called a system color. While a dual color printer requires only two images, a full color printer typically requires four images, one for each of the three primary colors of cyan, magenta, yellow, and an additional one for black. Color prints are currently produced by sequentially transferring and fusing overlapped system colors onto a single recording medium which is passed multiple times, once for each system color, through the printer. Such printers are referred to as multiple pass printers.
Multiple colors can be imprinted on a recording medium in one pass through the system by using a sequence of xerographic stations, one for each system color. If each station is associated with a separate photoreceptor, the printer is referred to as a multistation printer.
One example of multistation xerographic systems has a printer having multiple recording stations and multiple lens systems but a single shared ROS with only one rotating polygon mirror. The plurality of beams from the laser source are spatially overlapped using an optical beam combiner. The overlapped beams are then deflected using a single polygon mirror and the deflected beams are separating using an optical filter (and polarizers if more than two beams are used). The separated beams are directed onto associated photoreceptors.
One such multiple beam, single ROS xerographic printing system is disclosed in U.S. Pat. No. 5,243,359, commonly assigned as the present application and herein incorporated by reference. A raster output scanning system employs a rotating polygon mirror that simultaneously deflects a plurality of clustered, dissimilar wavelength laser beams having common optical axes and substantially common origins from common mirror surface areas. The clustered beams are subsequently separated by a plurality of static optical filters and are then directed onto associated photoreceptors of a multistation printer.
Another such multiple beam, single ROS xerographic printing system is disclosed in U.S. Pat. No. 5,343,224, commonly assigned as the present application and herein incorporated by reference. A raster output scanning system employs a rotating polygon mirror that simultaneously deflects a plurality of clustered, dissimilar wavelengths, dissimilar polarization state laser beams having common optical axes and substantially common origins from common mirror surface areas. The clustered beams are subsequently separated by a plurality of static polarization filters and are then directed onto associated photoreceptors of a multistation printer.
A combination of a multiple wavelength laser and post-emission polarizers presents still a third multiple beam, single ROS xerographic printing system, disclosed in U.S. Pat. No. 5,371,526, commonly assigned as the present application and herein incorporated by reference. A raster output scanning system employs a rotating polygon mirror that simultaneously deflects a plurality of clustered, dissimilar wavelength, dissimilar polarization state laser beams having common optical axes and substantially common origins from common mirror surface areas. The clustered beams are subsequently separated by a plurality of static optical filters and static polarization filters and are then directed onto associated photoreceptors of a multistation printer.
However, under all three prior art multiple beam, single ROS xerographic printing systems, at least one laser is required for each position or station. Thus, printing at four stations requires an array of four lasers, each with a different wavelength or polarization state. As the printing speed increases, larger arrays are needed for the multiple beam raster output scanning system in order to simultaneously expose each station with more than one beam. For example, dual beam printing at each of four stations requires eight lasers and quadbeam printing at each of four stations requires 16 lasers.
This progression shows that the number of lasers and therefore the physical size of the laser array becomes increasingly large leading to increasingly complex construction of the semiconductor laser array structure and to increasingly complex scanning optics to separate and locate the beams on the stations.
An architecture is needed for reducing the number of different lasers in a multiple beam, single ROS xerographic printing system. Similarly, an architecture is needed for a multiple beam, single ROS xerographic printing system in which the number of printing stations and laser beams used to print at each station is straightforwardly extensible.
It is an object of this invention to provide a multiple beam raster output scanning system with time multiplexed lasers and dynamic beam separators.